1. Field of the Invention
The present invention relates to an acceleration sensor utilizing a piezoelectric element, and more particularly, it relates to an acceleration sensor suitably integrated into an air bag system which is mounted on an automobile, for example.
2. Description of the Background Art
An air bag system which is carried on an automobile is driven in response to acceleration applied upon a collision or the like. In order to ensure the operation of such an air bag system, an acceleration sensor is generally integrated therein. In relation to such an acceleration sensor, there has been proposed a technique of employing a piezoelectric element which is deformed in response to acceleration applied thereto to output an electric signal, as disclosed in U.S. Pat. No. 4,700,973, for example.
An example of a well-known conventional acceleration sensor employing a piezoelectric element is now described with reference to FIGS. 16 and 17.
This acceleration sensor has a piezoelectric element 61 and a hybrid IC 62 including a circuit for processing a detection signal which is outputted from the piezoelectric element 61. An end of the piezoelectric element 61 is fixed to a support member 65 which is fixed onto a metal base plate 63. A free end of the piezoelectric element 61 is deflected by acceleration applied along a direction C which is orthogonal to the surface of the support member 65 supporting the piezoelectric element 61, to output a detection signal responsive to the degree of the acceleration.
The hybrid IC 62 is fixed onto the metal base plate 63. The piezoelectric element 51 and the hybrid IC 62 are electrically connected with each other through lead wires 66. A plurality of pin terminals 67 are mounted to downwardly extend from the metal base plate 63 for electrically connecting the acceleration sensor with an external unit. The piezoelectric element 61 and the hybrid IC 62 are sealed with a cap 64 which is fixed to the metal base plate 63.
In the aforementioned conventional acceleration sensor, the piezoelectric element 51 is supported by the support member 65 in a cantilever manner, so that the piezoelectric element 61 is bent/deformed in response to the acceleration applied thereto. When extremely large acceleration is applied, therefore, the free end portion of the piezoelectric element 61 is remarkably swung. Consequently, the piezoelectric element 61 itself may be broken when the same is exposed to extremely large acceleration or impact of about 2000 G, for example.
While the piezoelectric element 61 and the hybrid IC 62 are connected with each other by the plurality of lead wires 66 in the aforementioned acceleration sensor, the lead wires 66 may be broken due to metal fatigue caused by vibration. In order to prevent such disconnection, it may be effective to increase the diameters of the lead wires 66 thereby improving strength. In this case, however, the lead wires 66 may resonate with vibration following acceleration, to exert a bad influence on the acceleration detecting operation of the piezoelectric element 61.
In the aforementioned acceleration sensor detecting the acceleration along the direction C shown in FIG. 17, further, it is necessary to provide a strong structure in the portion supporting the piezoelectric element 61 so that this portion is not deflected by acceleration. Thus, the piezoelectric element 61 and the hybrid IC 62 are mounted on the metal base plate 63, which in turn closes an opening surface of the cap 64. In such a structure using the heavy and high-priced metal base plate 63, however, the overall acceleration sensor is increased in size while the acceleration sensor itself must be strongly mounted on a target of measurement, and hence the cost is extremely increased.